So the cornerstones of this project are: Be cheap but let it look great! No external power source, once it's on stage! Build it small, so you can build it in!
The starting point is the very small fog machine "tiny fogger", we already have in stock (sorry no cheap solution here) and from there on it was very straight forward to the prototype you will see here...
Step 1: Basics
So I thought, why not take the icepacks one uses in cooling boxes for camping, some fitting tupperware or similar and styrofoam? Add some leftover tube fittings and tape, and here we go:
Step 2: The Box
I drilled two holes into opposite sides of the box and glued in a fitting into each with some hot-glue.
Then I insulated everything with styrofoam.
This is a prototype or proof-of-concept. There probably will be a more elaborate version later on, when the play goes on stage. When it's done I will post some pictures.
Here's a video of the working prototype: